The invention generally relates to the field of protection circuits which prevent circuit failures due to excessive current and/or excessive voltage. The present invention has particular application to such protection circuits used by a microprocessor which monitors the output of an analog sensor wherein the microprocessor receives an analog sensor ground signal from the sensor in addition to a main power ground signal directly connected to the microprocessor's own internal ground reference potential.
In prior circuits, often the ground potential of a sensor is separate from the main system power ground potential. Thus microprocessors which effectively monitor the output of the sensor receive an analog sensor ground reference potential from the sensor in addition to receiving a power ground reference potential which corresponds to the main ground reference potential of the microprocessor. Typically the analog sensor ground potential and the main power ground potential are at substantially the same voltage potential and a low resistance connection is provided therebetween to insure this occurrence. In such prior systems, faults can occur resulting in the analog sensor ground potential substantially differing from the main power ground potential. This can occur, when such a system is in a battery powered automobile electronic environment, when the analog sensor ground terminal is accidently connected to a nongrounded battery potential. In such a case, a very large current is passed through the low resistance connection and any printed circuit board metalization paths which connect the low resistance connection between the main analog sensor ground terminal and the power ground terminal. This can result in burning up a printed circuit board metalization path, which is sometimes also referred to as a "trace", and providing a permanent open circuit connection whereas typically an effective short circuit connection is desired. In addition, a substantial voltage at the main analog sensor ground terminal may cause damage to the microprocessor regardless of the current which exists external to the microprocessor between the main analog sensor ground terminal and the power ground terminal. Some prior sensor monitoring systems prevent excessive current in the ground connections by providing a substantial resistance and/or offset voltages between the main analog sensor ground terminal and the power ground terminal, but such systems cause inaccuracies in measuring the voltage produced by the sensor.
Some prior protection circuits have utilized fuses or other types of circuit breakers to prevent excessive current caused by a ground fault. In general, other protection circuits, other than ground fault protection circuits, have utilized voltage limiting devices such as zener diodes so as to prevent excessive voltage. However, typically only current limiting protection circuits have been utilized to prevent excessive current between analog sensor ground and power ground connections. Thus failures caused by overvoltage during a ground fault condition may still occur. Also, typically such protection circuits which utilize relays, fuses or circuit breakers are expensive and do not provide for any rapid resuming of normal operation when a detected fault ceases to exist.